Generalized Bessel numbers.

Terms

a(0) =1a(1) =1a(2) =2a(3) =3a(4) =6a(5) =12a(6) =28a(7) =69a(8) =182a(9) =497a(10) =1399a(11) =4028a(12) =11852a(13) =35626a(14) =109494a(15) =344338a(16) =1108565a(17) =3653536a(18) =12320940a(19) =42483305a(20) =149640000a(21) =537975261a(22) =1972713660a(23) =7374794356a(24) =28100132482a(25) =109117922021a(26) =431821675389